Effects of Residual Solvent on Membrane Structure and Gas Permeation in a Polymer of Intrinsic Microporosity: Insight from Atomistic Simulation

Abstract

Microstructure and permeation in a polymer membrane are closely related to residual solvent during membrane fabrication. In this study, we conduct atomistic simulation to investigate the effects of residual solvent on free volume distribution and H2 permeation in a polymer of intrinsic microporosity (PIM-1). The interaction energies of three solvents (CHCl3, CH3OH, and H2O) with PIM-1 are predicted to be −16.3, −9.6, and −7.0 kcal/mol, respectively, which are in good agreement with available experimental data. On this basis, a structure–property relationship is proposed between interaction energy and the critical volume of solvent. The cyano and dioxane groups in PIM-1 interact preferentially with CH3OH and H2O; however, the carbon atoms interact more strongly with CHCl3. The mobility of residual solvent is found to decrease in the order of H2O > CH3OH > CHCl3. Upon comparison, the mobility of PIM-1 chains is smaller but facilitated by solvent due to the cooperative interactions between polymer and solvent. The fractional free volumes and large-size voids in PIM-1/solvent membranes are observed to decrease as CH3OH > CHCl3 > H2O, consistent with positron annihilation lifetime spectroscopy measurements. The solubility and diffusion coefficients of H2 decrease in the same hierarchy, and the predicted and experimental coefficients are in fairly good agreement. This simulation study provides atomistic insight into the microscopic properties of residual solvent in a polymer membrane and reveals the crucial role of residual solvent in tailoring membrane structure and gas permeation.